The development of oil and gas fields in the Arctic had encouraged a search for structural steels having good low-temperature properties for such applications as line pipe, line-pipe fittings and critical bridge members. The low-cost carbon and high strength, low-alloy steels currently used for these applications in warmer environments do not have the desired toughness at low temperatures in section thicknesses of about 1 to 2 inches. For such Arctic applications, it will be necessary that the structural steel have a minimum yield strength of at least 60 ksi, and good impact toughness down to temperatures as low as -80.degree.F.
Although many low-alloy and alloy steels are known which have excellent low temperature properties, more than sufficient to meet the above requirements, such as the "T-1" steels and the 3 to 9% nickel cryogenic steels, these prior art steels provide properties far in excess of those desired and are therefore too expensive for high tonnage applications such as line pipe. In addition, many of these steels are quenched and tempered martensitic grades many of which are difficult to weld in the field.